Chair back



FIG. 1 is a rear view of the chair back shown in use;

FIG. 2 is a perspective view;

FIG. 3 is a front view;

FIG. 4 is a back view;

FIG. 5 is a left side view;

FIG. 6 is a top plan view; and,

FIG. 7 is a bottom plan view.

The broken lines shown are included for illustrative purposes only form no part of the claimed design. 

The ornamental design for chair back, as shown and described. 